Certain scientific experiments, especially neutron diffraction or scattering, require a sample to be subjected to a magnetic field that is orientable in all the directions of space. The sample holder is a cryogenic chamber, which entails the field source comprising a free access usually a circular cylinder of vertical axis. A free space must also be arranged on either side of the horizontal plane passing through the sample, in order to allow the passage of neutron beams having a certain angular aperture. Lastly, the field generating system must remain stationary, the variable orientation of the field being obtained by adjusting supply currents.
Typically, it is desired to generate, in a 10 mm-diameter sphere, an orientable magnetic field having an amplitude of about 1 T and a uniformity of better than 5% or even 1%, while leaving an axial access that is at least 100 mm in diameter, and providing an equatorial angular aperture of about 2.5°.
An orientable magnetic field having the required amplitude and uniformity characteristics may be generated by means of the device in FIG. 1, which comprises three pairs of coaxial coils (or split coils) SCx, SCY, SCz, the axes of which are orthogonal and cross at the centre of the sample. Such a device has been installed in the (British) ISIS muon and neutron source, allowing a magnetic field uniformity better than 0.5% to be obtained in a 10 mm-diameter sphere, but its angular aperture is very limited. In particular, it does not have a continuous equatorial aperture, but only relatively narrow accesses along the axes x, y and z.
A related problem is that of how to generate a rotating magnetic field (in a plane or a cone, i.e. having a rotating radial component and a constant axial component), rotating magnetic fields being important for nuclear magnetic resonance in particular. Accessibility requirements are also present in these applications. In the prior art (see for example document US 2008/0024130) a rotating magnetic field is obtained by making a magnet rotate, which poses problems with regard to complexity, reliability and vibrations.